This invention relates to compliant fluid film bearings, and more particularly to a compliant hydrodynamic fluid film journal bearing. The invention is related to copending application Ser. No. 080,503 of Hooshang Heshmat entitled "Stepped, Split, Cantilevered Compliant Bearing Support" filed concurrently herewith, the disclosure of which is incorporated by reference herein.
Compliant hydrodynamic gas journal bearings are being used in high performance machinery which is subject to extreme conditions of temperature and speed. These bearings are ideally suited for these conditions because they do not suffer from the speed and durability limitations of rolling element bearings, and because they do not require oil lubrication and therfore do not require oil circulation, seals, and conditioning equipment necessary for use in oil bearings.
A compliant hydrodynamic fluid film journal bearing ordinarily employs one or more bearing pad assemblies, each including a resilient supporting element and an attached overlying bearing sheet. The pads are mounted on the inside surface of a journal sleeve in bearing relationship to a relatively rotating shaft. According to the theory of the hydrodynamic compliant fluid film bearing, a hydrodynamic supporting fluid film is generated by the relative movement of the rotating shaft over the bearing sheet of the pad to support the shaft or journal on a thin cushion of fluid. The compliance of the supporting element underlying the bearing sheet enables it to deflect to assume a profile relative to the shaft bearing surface which is productive of a supporting pressurized fluid cushion over the pad. It also enables the bearing sheet to conform, to some extent, to misaligned, unbalanced, and thermally or mechanically distorted shafts.
Despite the proven advantages that the use of these bearings confer, we have identified certain situations in which room for improvement exists. For example, high load carrying capacity for existing compliant gas bearings is generated only at relatively high speeds. Normally this is not a matter of concern because gas bearings are normally used in high speed applications. However, it would militate for longer bearing life if load capacity were generated at a lower speed to reduce the period during start-up and slow-down that the rotating member is in contact with the bearing sheet. A high load capacity at low rotor speeds would also increase the range of applications for which these bearings are applicable.
Some gas bearings which are designed for a particular range of speed, load, and temperature lose their load capacity when these ranges are exceeded. The desirability of bearings which function well at conditions considerably in excess of the design point, as well as the design point conditions, would increase the usefulness of these bearings by increasing their range of application and their safety margin, and thereby lessening the expense for redundant or larger capacity bearings.
There are certain circumstances in which some compliant journal bearings can exhibit an instability phenomenon. For example, certain designs of single pad journal bearings operating under light load at high speed and without external damping have encountered this problem. When it occurs, the effect is a substantial increase in the amplitude of the rotor gyration orbit which could ultimately cause damage to the bearing or the rotor.